HTTP Monitoring pool some times problem

In this case maybe the problem come from your application who do not respond correctly to the monitor's request during a lapse of time.

In order to validate this possibility you can enable "Monitor Logging":

https://support.f5.com/csp/article/K12531

to enable monitoring logging go in the pool then click on the poolmemeber, you will then have the possibility to activate this feature.

The BIG-IP system generates the following log file for the pool member:

/var/log/monitors/....

you could check the logs and see why your Backend server was down. by controlling the response of the server ...

keep me in touch if you need more details.

regards,